An optoelectronic device (e.g. an organic light emitting diode (OLED), for example a white organic light emitting diode (WOLED), a solar cell, etc.) on an organic basis is usually distinguished by its mechanical flexibility and moderate production conditions. Compared with a device composed of inorganic materials, an optoelectronic device on an organic basis can be produced potentially cost-effectively on account of the possibility of large-area production methods (e.g. roll-to-roll production methods).
A WOLED consists e.g. of an anode and a cathode with a functional layer system therebetween. The functional layer system consists of one or a plurality of emitter layer/s, in which the light is generated, one or a plurality of charge generating layer structure/s each composed of two or more charge generating layers (CGL) for generating charge carriers, and one or a plurality of electron blocking layers, also designated as hole transport layer(s) (HTL), and one or a plurality of hole blocking layers, also designated as electron transport layer(s) (ETL), in order to direct the current flow.
In the simplest embodiment, the charge generating layer structure conventionally consists of a hole-conducting charge generating layer and an electron-conducting charge generating layer, which are directly connected to one another, with the result that illustratively a pn junction is formed. In the pn junction, a depletion region is formed, in which electrons of the hole-conducting charge generating layer migrate into the electron-conducting charge generating layer, wherein the electron-conducting charge generating layer is an n-doped charge generating layer. As a result of a voltage being applied to the pn junction in the reverse direction, in the depletion region electrons and holes are generated which migrate into the emitter layers and can generate electromagnetic radiation as a result of recombination (e.g. light).
An OLED can be produced with good efficiency and lifetime by means of conductivity doping by the use of a p-i-n (p-doped-intrinsic-n-doped) junction analogously to the conventional inorganic LED. In this case, the charge carriers from the p-doped and respectively n-doped layers are injected in a specific manner into the intrinsic layer, in which the excitons, i.e. electron-hole pairs, are formed.
By stacking one or a plurality of intrinsic layers one above another, it is possible to obtain in the OLED, with practically the same efficiency and identical luminance, significantly longer lifetimes compared with an OLED including only one intrinsic layer. For the same current density, double to triple the luminance can thus be realized. For the stacking one above another, charge generating layers consisting of a highly doped pn junction are required.
The hole-conducting and electron-conducting charge generating layers can each consist of one or a plurality of organic and/or inorganic substance(s). In the production of the charge generating layer, the respective matrix is usually admixed with one or a plurality of organic or inorganic substances (dopants) in order to increase the conductivity of the matrix. This doping can produce electrons (n-doped; dopants e.g. metals having a low work function, e.g. Na, Ca, Cs, Li, Mg or compounds thereof e.g. Cs2CO3, Cs3PO4, or organic dopants from the company NOVALED, e.g. NDN-1, NDN-26) or holes (p-doped; dopant e.g. transition metal oxides, e.g. MoOx, WOx, VOx, organic compounds, e.g. Cu(I)pFBz, F4-TCNQ, or organic dopants from the company NOVALED, e.g. NDP-2, NDP-9) as charge carriers in the matrix.
The use of a CGL in an optoelectronic device presupposes a simple construction, i.e. as few layers as possible, which are as easy as possible to produce. Furthermore, a small voltage drop across the CGL and the highest possible transmission of the CGL layers are necessary, i.e. the lowest possible absorption losses in the spectral range, of the electromagnetic radiation emitted by the OLED.
In a manner similar to inorganic layers at high temperatures in the manufacture of semiconductor devices, for example at temperatures of greater than 200° C., during manufacture and during operation even at temperatures of less than 100° C. material of the organic layers can diffuse into other layers (partial layer interdiffusion), e.g. parts of the n-doped charge generating layer into the p-doped charge generating layer of a charge generating layer structure in an OLED.
When an electric field is applied to the charge generating layer structure, a voltage drop across this layer structure is measurable by means of the layer interdiffusion. Said voltage drop increases with the operating period, since the diffusion of conductive organic substances is directed in an electric field. This can limit the life of the operating period of organic optoelectronic devices.
In order to suppress the partial layer interdiffusion (barrier effect), an interlayer can be inserted between the individual organic layers. In this case, the interlayer prevents the layer interdiffusion, for example of the dopant or of the matrix substance. Furthermore, the interlayer can prevent a reaction of the first electron-conducting charge generating layer with the second electron-conducting charge generating layer, i.e. the interlayer forms a reaction barrier. Furthermore, the interlayer can reduce the interfacial roughness between the hole-conducting charge generating layer and the electron-conducting charge generating layer by the surface roughness of the electron-conducting charge generating layer being reduced or compensated for by means of the interlayer.
However, the interlayer constitutes an optoelectronic resistance in the charge generating layer structure and can reduce the efficiency of the optoelectronic device. The optoelectronic resistance of a layer, in various embodiments, can be understood to mean an absorption of electromagnetic radiation, for example light, in the layer and an electrical resistance, for example as a result of a voltage drop across said layer.